America’s Children and the Environment Biomonitoring Methods Lead

Biomonitoring: Lead

Methods

Indicator

Bl. Lead in children ages 1 to 5 years: Median and 95th percentile concentrations in blood, 1976-
2018

B2. Lead in children ages 1 to 5 years: Median concentrations in blood, by race/ethnicity and
family income, 2015-2018

Summary

Since the 1970s, the National Center for Health Statistics, a division of the Centers for Disease
Control and Prevention, has conducted the National Health and Nutrition Examination Surveys
(NHANES), a series of U.S. national surveys of the health and nutrition status of the non-
institutionalized civilian population. The National Center for Environmental Health at CDC
measures environmental chemicals in blood and urine samples collected from NHANES
participants.1 Indicators Bl and B2 use blood lead measurements in children ages 1 to 5 years.
NHANES II (1976-1980) included blood lead data for children from six months to 5 years.
NHANES III (1988-1994) and the 1999-2018 surveys included blood lead data for children ages
1 to 5 years11.

Indicator Bl is the median and 95th percentile concentrations of blood lead for children ages 1 to
5 years for 1976-2018. The median is the estimated concentration such that 50% of all non-
institutionalized civilian children ages 1 to 5 years have blood lead concentrations below this
level. The 95th percentile is the estimated concentration such that 95% of all non-institutionalized
civilian children ages 1 to 5 years have blood lead concentrations below this level. Indicator B2
is the median concentrations of blood lead for children ages 1 to 5 years for 2015-2018, stratified
by race/ethnicity and family income.

Supplementary Tables: Table B la presents the median and 95th percentile concentrations of
blood lead for children ages 1 to 17 years for 2017-2018, stratified by age group.

Table B2a presents the 95th percentile concentrations of blood lead for children ages 1 to 5 years
for 2015-2018, stratified by race/ethnicity and family income. Table B2b presents the median
concentrations of blood lead for children ages 1 to 5 years for 1991-1994, stratified by
race/ethnicity and family income. The survey data were weighted to account for over-sampling,
non-response, and non-coverage.

1 Centers for Disease Control and Prevention. 2009. Fourth National Report on Human Exposure to Environmental
Chemicals. Atlanta, GA. Available at: www.cdc.qov/exposurereport.

" We are grateful to Sheila Franco, MS, Captain Greg Hales, MD, of the CDCand Joseph Afful, MS of Peraton
Corporation for their assistance in compiling and analyzing the blood lead data.

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Data Summary

Indicator Bl. Lead in children ages 1 to 5 years: Median and 95th percentile concentrations in blood, 1976-2018

Data

Bl

ood leac



Years

1976-

1988-

1991-

1999-

2001-

2003-

2005-

2007-

2009-

2011-

2013-

2015-

2017-



1980

1991

1994

2000

2002

2004

2006

2008

2010

2012

2014

2016

2018

Limits of

Not
reported











0.3 or
0.25













Detection
(lig/dL)*

1

1

0.3

0.3

0.3

0.25

0.25

0.25

0.07

0.07

0.07

Number of Values

3,762

3,043

2,744

1,085

1,330

1,267

1,410

1,231

1,295

1,203

1,131

1,144

931

Number of Non-
Missing Values**

2,360
(63%)

2,232
(73%)

2,392
(87%)

723
(67%)

898
(68%)

911
(72%)

968
(69%)

817

(66%)

836
(65%)

713
(59%)

818
(72%)

790
(69%)

629
(68%)

Number of

1,402

811

352

362

432

356

442

414

459

490

313

354

302

Missing Values**

(37%)

(27%)

(13%)

(33%)

(32%)

(28%)

(31%)

(34%)

(35%)

(41%)

(28%)

(31%)

(32%)

Percentage Below
Limit of



4

8

0

1

0

0

0

0

1

0

0

0

Detection***



























* The Limit of Detection (LOD) is defined as the level at which the measurement has a 95% probability of being greater than zero.

**Non-missing values include those below the analytical LOD, which are reported as LOD/V2. Missing values are the number of sampled children ages 1 to 5 years in the Mobile
Examination Center (MEC) sample or subsample that have no value reported for the particular variable used in calculating the indicator.

"This percentage is survey-weighted using the NHANES MEC survey weights for the given period.

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National Health and Nutrition Examination Surveys (NHANES)

Indicators B1 and B2 use blood lead measurements in children ages 5 and under. NHANES II
(1976-1980) included blood lead data for children from six months to 5 years. NHANES III
(1988-1994) and the NHANES 1999-2000, 2001-2002, 2003-2004, 2005-2006, 2007-2008,
2009-2010, 2011-2012, 2013-2014, 2015-2016 and 2017-2018 surveys included blood lead data
for children ages 1 to 5 years. The NHANES data were obtained from:

http://www.cdc.gov/nchs/nhanes.htm As an exception, summary statistics based on the restricted
access blood lead data for children ages 1 to 5 years in 2017-2018 were provided directly by
CDC. Following the CDC recommended approach, values below the analytical limit of detection
(LOD) were replaced by LOD/V2,'"

The NHANES use a complex multi-stage, stratified, clustered sampling design. Certain
demographic groups were deliberately over-sampled, including Mexican-Americans, Blacks,
and, from 2007 onwards, All Hispanics, then, from 2011 onwards, Asians, to increase the
reliability and precision of estimates of health status indicators for these population subgroups.
The publicly released data includes survey weights to adjust for the over-sampling, non-
response, and non-coverage. The statistical analyses used the applicable MEC survey weights
(N2LB0300 for NHANES II, WTPFHX1 for Phase I of NHANES III, WTPFHX2 for Phase 2 of
NHANES III, WTPFH6 for the Total sample of NHANES III, WTMEC2YR for 1999 to 2012,
WTSH2YR for 2013-2016, and WTMEC2YR for 2017-2018) to re-adjust the blood lead data to
represent the national population.

Race/Ethnicity and Family Income

For Indicator B2, the percentiles were calculated for demographic strata defined by the
combination of race/ethnicity and family income.

The family income was characterized based on the INDFMPIR variable, which is the ratio of the
family income to the poverty level. The National Center for Health Statistics used the U.S.
Census Bureau Current Population Survey definition of a "family" as "a group of two people or
more (one of whom is the householder) related by birth, marriage, or adoption and residing
together" to group household members into family units, and the corresponding family income

111 See Hornung RW, Reed LD. 1990. Estimation of average concentration in the presence of nondetectable values.

Appl Occup Environ Hvg 5:46-51.

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for the respondent was obtained during the interview. The U.S. Census Bureau defines annual
poverty level money thresholds varying by family size and composition. The poverty income
ratio (PIR) is the family income divided by the poverty level for that family. Family income was
stratified into the following groups:

• Below Poverty Level: PIR < 1

• Above Poverty Level: PIR > 1

• Unknown Income: PIR is missing

For the four year period 2015-2018, the weighted percentage of children ages 1 to 5 years with
unknown income was 9%.

Race/ethnicity was characterized using the RIDRETH1 variable. The possible values of this
variable are:

• 1. Mexican American

• 2. Other Hispanic

• 3. Non-Hispanic White

• 4. Non-Hispanic Black

• 5. Other Race - Including Multi-racial

• Missing

Category 5 includes: all Non-Hispanic single race responses other than White or Black; and
multi-racial responses.

For indicator B2, the RIDRETH1 categories 2, 5, and missing were combined into a single "All
Other Races/Ethnicities" category. This produced the following categories:

• White non-Hispanic: RIDRETH1 = 3

• Black non-Hispanic: RIDRETH1 = 4

• Mexican-American: RIDRETH1 = 1

• All Other Races/Ethnicities: RIDRETH1 = 2 or 5 or missing

The "All Other Races/Ethnicities" category includes multiracial persons and individuals whose
racial or ethnic identity is not White non-Hispanic, Black non-Hispanic, or Mexican-American.
Except for non-Mexican-American Hispanics in 2007- 2018 and Asian non-Hispanics in 2011-
2018, persons of "All Other Races/Ethnicities" are selected into the survey with a probability
that is very much lower than White non-Hispanic, Black non-Hispanic and Mexican-American
individuals, and as a group they are not representative of all other race and ethnicities in the
United States.

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Calculation of Indicator

Indicator B1 is the median and 95th percentile for blood lead in children of ages 1 to 5 years.
Supplemental Table Bla presents the median and 95th percentile for blood lead in children of
ages 1 to 17 years, stratified by age group. Indicator B2 is the median of blood lead in children of
ages 1 to 5 years, stratified by race/ethnicity and family income. Supplemental Table B2a
presents the 95th percentile of blood lead in children of ages 1 to 5 years, stratified by
race/ethnicity and family income. Supplemental Table B2b presents the median of blood lead in
children of ages 1 to 5 years, stratified by race/ethnicity and family income for 1991-1994. The
median is the estimated concentration such that 50% of all non-institutionalized civilian children
ages 1 to 5 years have blood lead concentrations below this level. The 95th percentile is the
estimated concentration such that 95% of all non-institutionalized civilian children ages 1 to 5
years have blood lead concentrations below this level.

To simply demonstrate the calculations, we will use the NHANES 2009-2010 blood lead values
for children ages 1 to 5 years of all race/ethnicities and all incomes as an example. We have
rounded all the numbers to make the calculations easier:

We begin with all the non-missing NHANES 2009-2010 blood lead values for children ages 1 to
5 years. Assume for the sake of simplicity that valid data on blood lead were available for every
sampled child. Each sampled child has an associated survey weight, WTMEC2YR, that
estimates the annual number of U.S. children represented by that sampled child. For example, the
lowest blood lead measurement for a child between 1 and 5 years of age is 0.18 (j,g/dL with a
survey weight of 30,000, and so represents 30,000 children between 1 and 5 years of age. The
total of the survey weights for the sampled children equals 20 million, the total U.S. population
of children between 1 and 5 years of age. The second lowest measurement is 0.27 (j,g/dL with a
survey weight of 40,000, and so represents another 40,000 U.S. children between 1 and 5 years
of age. The highest measurement was 17 (j,g/dL, with a survey weight of 13,000, and so
represents another 13,000 U.S. children between 1 and 5 years of age.

To calculate the median, we can use the survey weights to expand the data to the entire U.S.
population of 20 million children ages 1 to 5 years. We have 30,000 values of 0.18 |ig/dL from
the lowest measurement, 40,000 values of 0.27 |ig/dL from the second lowest measurement, and
so on, up to 13,000 values of 17 |ig/dL from the highest measurement. Arranging these 20
million values in increasing order, the 10 millionth value is 1.15 (J,g/dL. Since half of the values
are below 1.15 and half of the values are above 1.15, the median equals 1.15 (j,g/dL. To calculate
the 95th percentile, note that 95% of 20 million equals 19 million. The 19 millionth value is 3.4
[j,g/dL. Since 95% of the values are below 3.4, the 95th percentile equals 3.4 (j,g/dL.

In reality, the calculations need to take into account that blood lead measurements were not
available for every respondent, and to use exact rather than rounded numbers. There were blood
lead measurements for only 836 of the 1,295 sampled children ages 1 to 5 years. The survey
weights for all 1,295 sampled children add up to 21.1 million, the U.S. population of children
ages 1 to 5 years. The survey weights for the 836 sampled children with blood lead data add up
to 13.5 million. Thus, the available data represent 13.5 million values and so represent only 64%
of the U.S. population of children ages 1 to 5 years. The median and 95th percentiles are given by

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the 6.75 millionth (50% of 13.5 million) and 12.83 millionth (95% of 13.5 million) U.S. child's
value. These calculations assume that the sampled children with valid blood lead data are
representative of the children without valid blood lead data.

Equations

These percentile calculations can also be given as the following mathematical equations, which
are based on the default percentile calculation formulas from Statistical Analysis System (SAS)
software. Exclude all missing blood lead values. Suppose there are n children of ages 1 to 5 years
with valid blood lead values. Arrange the blood lead concentrations in increasing order
(including tied values) so that the lowest concentration is x(l) with a survey weight of w(l), the
second lowest concentration is x(2) with a survey weight of w(2), ..., and the highest
concentration is x(n) with a survey weight of w(n).

1.	Sum all the survey weights to get the total weight W:

W = E[1 < i < n] w(i)

2.	Find the largest number i so that the total of the weights for the i lowest values is less than or
equal to W/2.

E[j < i] w(j) < W/2 < E[j < i + 1] w(j)

3.	Calculate the median using the results of the second step. We either have

E[j < i] w(j) = W/2 < E[j < i + 1] w(j)

or

E[j < i] w(j) < W/2 < E[j < i + 1] w(j)

In the first case we define the median as the average of the i'th and i + l'th values:

Median = [x(i) + x(i + l)]/2 if S[j < i] w(j) = W/2

In the second case we define the median as the i + l'th value:

Median = x(i + 1) if E[j < i] w(j) < W/2

(The estimated median does not depend upon how the tied values of x(j) are ordered).

A similar calculation applies to the 95th percentile. The first step to calculate the sum of the
weights, W, is the same. In the second step, find the largest number i so that the total of the
weights for the i lowest values is less than or equal to 0.95W.

E[j < i] w(j) < 0.95W < E[j < i + 1] w(j)

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In the third step we calculate the 95th percentile using the results of the second step. We either
have

E[j < i] w(j) = 0.95W < E[j < i + 1] w(j)

or

E[j < i] w(j) < 0.95W < E[j < i + 1] w(j)

In the first case we define the 95th percentile as the average of the i'th and i + l'th values:

95th Percentile = [x(i) + x(i + l)]/2 if S[j < i] w(j) = 0.95W
In the second case we define the 95th percentile as the i + l'th value:

95th Percentile = x(i + 1) if S[j < i] w(j) < 0.95W

Relative Standard Error

The uncertainties of the median and 95th percentile values were calculated using a revised
version of the CDC method given in CDC 2005,1V Appendix C, and the SAS® program provided
by CDC. The method uses the Clopper-Pearson binomial confidence intervals adapted for
complex surveys by Korn and Graubard (see Korn and Graubard, 1999,v p. 65). The following
text is a revised version of the Appendix C.

Step 1: Use SAS® Proc Univariate to obtain a point estimate Psas of the percentile value. Use the Weight
option to assign the exact correct sample weight for each chemical result.

Step 2: Use SUDAAN® Proc Descript with Taylor Linearization DESIGN = WR (i.e.,
sampling with replacement) and the proper sampling weight to estimate the proportion (p) of subjects with
results less than and not equal to the percentile estimate Psas obtained in Step 1 and to obtain the standard
error (sep) associated with this proportion estimate. Compute the degrees-of-freedom adjusted effective
sample size

n,;lf = ( tnum/ tdenom )2 p (1 - p) / (sep 2)

where tri,m, and tdenom are 0.975 critical values of the Student's t distribution with degrees of freedom
equal to the sample size minus 1 and the number of primary sampling units (PSUs) minus the number of
strata, respectively. Note: the degrees of freedom for tdenom can vary with the demographic sub-group of
interest.

Step 3: After obtaining an estimate of p (i.e., the proportion obtained in Step 2), compute the Clopper-
Pearson 95% confidence interval (PiXx.rvif), Pu(x,ndf)) as follows:

PL(x,ndf) = ViFvi,v2 (0.025)/(v2 + ViFvi,v2(0.025))

IV CDC Third National Report on Human Exposure to Enviromnental Chemicals. 2005
v Korn E. L„ Graubard B. I. 1999. Analysis of Health Surveys. Wiley.

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Pu(x,n,;if) = v3Fv3,v4 (0.975)/(v4 + v3Fv3,v4(0.975))

where x is equal to p times ndf, vi = 2x, V2 = 2(ndf - x + 1), v3 = 2(x + 1), V4 = 2(ndf - x), and F
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with caution. Percentiles with a relative standard error that is 40% or greater, or without an
estimated relative standard error, or with 6 or less degrees of freedom, were treated as being
unreliable; these values were not tabulated and were flagged as having a large uncertainty.

Questions and Comments

Questions regarding these methods, and suggestions to improve the description of the methods,
are welcome. Please use the "Contact Us" link at the bottom of any page in the America's
Children and the Environment website.

Statistical Comparisons

Statistical analyses of the percentiles were used to determine whether the differences between
percentiles for different demographic groups were statistically significant. For these analyses, the
percentiles and their standard errors were calculated for each combination of age group, sex,
income group (below poverty, at or above poverty, unknown income), and race/ethnicity group
using the method described in the "Relative Standard Error" section. In the notation of that
section, the percentile and standard error are the values of Pcdc and Standard Error (Pcdc),
respectively. These calculated standard errors account for the survey weighting and design. For
these statistical comparisons, the calculations with data from NHANES III (1988-1994) used
survey weights, pseudo-strata, and pseudo-primary sampling unit values for the total sample (i.e.,
1988-1994) and not for the individual phases (i.e., 1988-1991 and 1991-1994). However, the
comparisons between 1991-1994 and 2015-2018 (Table 4) used the survey weights, pseudo-
strata, and pseudo-primary sampling unit values for Phase 2 only (1991-1994).

Using a weighted linear regression model, the percentile was assumed to be the sum of
explanatory terms for age, sex, income and/or race/ethnicity and a random error term; the error
terms were assumed to be approximately independent and normally distributed with a mean of
zero and a variance equal to the square of the standard error. In this model, the weight is the
inverse of the variance, so that percentiles with larger standard errors are given less of a
statistical weight in the fitted regression model. Using this model, the difference in the value of a
percentile between different demographic groups is statistically significant if the difference
between the corresponding sums of explanatory terms is statistically significantly different from
zero. A p-value at or below 0.05 implies that the difference is statistically significant at the 5%
significance level. No adjustment is made for multiple comparisons.

For each type of comparison, we present unadjusted and adjusted analyses. The unadjusted
analyses directly compare a percentile between different demographic groups. The adjusted
analyses add other demographic explanatory variables to the statistical model and use the
statistical model to account for the possible confounding effects of these other demographic
variables. For example, the unadjusted race/ethnicity comparisons use and compare the
percentiles between different race/ethnicity pairs. The adjusted race/ethnicity comparisons use
the percentiles for each age/sex/income/race/ethnicity combination. The adjusted analyses add
age, sex, and income terms to the statistical model and compare the percentiles between different

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race/ethnicity pairs after accounting for the effects of the other demographic variables. For
example, if White non-Hispanics tend to have higher family incomes than Black non-Hispanics,
and if the blood lead level strongly depends on family income only, then the unadjusted
differences between these two race/ethnicity groups would be significant but the adjusted
difference (taking into account income) would not be significant.

Comparisons between pairs of race/ethnicity groups and between income groups are shown in
Tables 1 and 2, respectively, for children ages 1 to 5 years. In Table 1, for the unadjusted "All
incomes" comparisons, the only explanatory variables are terms for each race/ethnicity group.
For these unadjusted comparisons, the statistical tests compare the percentiles for each pair of
race/ethnicity groups. For the adjusted "All incomes (adjusted for age, sex, income)"
comparisons, the explanatory variables are terms for each race/ethnicity group together with
terms for each age, sex, and income group. For these adjusted comparisons, the statistical test
compares the pair of race/ethnicity groups after accounting for any differences in the age, sex
and income distributions between the race/ethnicity groups.

In Table 1, for the unadjusted "Below Poverty Level" and "At or Above Poverty Level"
comparisons, the only explanatory variables are terms for each of the twelve
race/ethnicity/income combinations (combinations of four race/ethnicity groups and three
income groups). For example, in row 1, the p-value for "Below Poverty Level" compares White
non-Hispanics below the poverty level with Black non-Hispanics below the poverty level. The
same set of explanatory variables are used in Table 2 for the unadjusted comparisons between
one race/ethnicity group below the poverty level and the same race/ethnicity group at or above
the poverty level. The corresponding adjusted analyses include extra explanatory variables for
age and sex, so that race/ethnicity/income groups are compared after accounting for any
differences due to age or sex. Although these comparisons only involve the two income groups
with known incomes, these statistical models were fitted to all three income groups (including
those with unknown income) to make a more general, better fitting model; this approach has no
impact on the unadjusted p-values but has a small impact on the adjusted p-values. Also in Table
2, the unadjusted p-value for the population "All" compares the percentiles for children ages 1 to
5 years below poverty level with those at or above poverty level, using the explanatory variables
for the two income groups (below poverty, at or above poverty), excluding children with
unknown income. The adjusted p-value includes adjustment terms for age, sex, and
race/ethnicity in the model.

Additional comparisons are shown in Table 3. Comparisons are shown for differences between
age groups among children ages 1 to 17 years, between children ages 1 to 5 years below poverty
and those at or above poverty, and for changes over time (trends) for children ages 1 to 5 years.
The Against = "age" unadjusted p-value compares the percentiles for children between the age
groups 1, 2, 3-5, 6-10, 11-15, and 16-17 years, using the explanatory variables for these six age
groups. The adjusted p-value includes adjustment terms for sex, race/ethnicity, and income in the
model. The Against = "income" unadjusted p-value compares the percentiles for children ages 1
to 5 years below poverty level with those at or above poverty level, using the explanatory
variables for the two income groups (below poverty, at or above poverty). The adjusted p-value
includes adjustment terms for age, sex, and race/ethnicity in the model. The Against = "year" p-
value examines whether the linear trend in the percentiles for children ages 1 to 5 years is

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statistically significant (using the percentiles for each NHANES period regressed against the
midpoint of that period); the adjusted model for trend adjusts for demographic changes in the
populations from year to year by including terms for age, sex, income, and race/ethnicity.

Table 4 shows comparisons between blood lead levels in children ages 1 to 5 years in 1991-1994
and the blood lead levels in children ages 1 to 5 years in 2015-2018. The Against = "year" p-
value examines whether the change in the percentiles is statistically significant (using the
percentiles for the periods 1991-1994 and 2015-2018 regressed against the midpoints of those
two periods); the adjusted model adjusts for demographic changes in the populations from year
to year by including terms for age, sex, income, and race/ethnicity. The rows where the Subset
value is not missing show the p-values for different race/ethnicity groups.

The age groups used for the analyses of children ages 1 to 5 years were 1, 2, 3, 4, and 5.

For more details on these statistical analyses, see the memorandum by Cohen (2010).V1

vl Cohen, J. 2010. Selected statistical methods for testing for trends and comparing years or demographic groups in
ACE NHIS and NHANES indicators. Memorandum submitted to Dan Axelrad, EPA, 21 March, 2010.

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Table 1. Statistical significance tests comparing the percentiles of blood lead levels in children ages 1 to 5 years, between pairs of
race/ethnicity groups, for 2015-2018.



P-VALUES

Variable

Percentile

First
race/ethnicity
group

Second
race/ethnicity
group*

All

income

s

All

incomes
(adjusted
for age,

sex,
income)

Below
Poverty
Level

Below
Poverty
Level
(adjusted
for age,
sex)

At or
Above
Poverty
Level

At or
Above
Poverty
Level
(adjusted
for age,
sex)

lead

50

White non-
Hispanic

Black non-
Hispanic

0.001

0.010

0.192

0.098

0.070

0.041

lead

50

White non-
Hispanic

Mexican
American

0.131

0.375

0.771

0.234

0.503

0.935

lead

50

White non-
Hispanic

Other

0.719

0.002

0.440

0.034

0.575

0.034

lead

50

Black non-
Hispanic

Mexican
American

0.061

0.002

0.037

0.032

0.290

0.031

lead

50

Black non-
Hispanic

Other

<0.001

<0.001

0.010

0.003

0.009

<0.001

lead

50

Mexican
American

Other

0.038

0.026

0.529

0.315

0.203

0.044

lead

95

White non-
Hispanic

Black non-
Hispanic

0.744

<0.001

0.566

<0.001

0.276

<0.001

lead

95

White non-
Hispanic

Mexican
American

0.219

0.808

0.618

<0.001

0.123

0.676

lead

95

White non-
Hispanic

Other

0.763

<0.001

0.691

<0.001

0.322

0.152

lead

95

Black non-
Hispanic

Mexican
American

0.008

<0.001

0.252

<0.001

0.640

<0.001

lead

95

Black non-
Hispanic

Other

0.302

<0.001

0.773

0.057

0.694

<0.001

lead

95

Mexican
American

Other

0.051

<0.001

0.286

<0.001

0.085

0.003

* "Other" represents the "All Other Races/Ethnicities" category, which includes all other races and ethnicities not specified, together with those individuals who report more than one race.

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Table 2. Statistical significance tests comparing the percentiles of blood lead levels in children ages 1 to 5 years, between those below
poverty level and those at or above poverty level, for 2015-2018.



P-Values for difference between
income levels

Variable

Percentile

Population*

Unadjusted

Adjusted (for age,
sex)**

Lead

50

All

0.004

<0.001

Lead

50

White non-Hispanic

0.167

<0.001

Lead

50

Black non-Hispanic

0.003

0.068

Lead

50

Mexican American

0.294

0.167

Lead

50

Other

0.316

0.161

Lead

95

All

0.285

<0.001

Lead

95

White non-Hispanic

0.991

0.465

Lead

95

Black non-Hispanic

0.052

0.061

Lead

95

Mexican American

0.233

0.001

Lead

95

Other

0.013

<0.001

*	"Other" represents the "All Other Races/Ethnicities" category, which includes all other races and ethnicities not specified, together with those individuals who report more than one race.

*	* Comparison for "All" is adjusted for age, sex, and race/ethnicity; comparisons for race/ethnicity categories are adjusted for age and sex.

Table 3. Other statistical significance tests comparing the percentiles of blood lead levels in children by age for ages 1 to 17 years for
2017-2018, by income for ages 1 to 5 years for 2015-2018, and trends for children ages 1 to 5 years for 1988-2018 and for 1999-2018.



P-VALUES

Variable

Percentile

From

To

Ages

Against

Unadjusted

Adjusted*

Lead

50

2017

2018

1 to 17 years

age

<0.001

<0.001

Lead

50

2015

2018

1 to 5 years

income

0.004

<0.001

Lead

50

1988

2018

1 to 5 years

year

<0.001

<0.001

Lead

50

1999

2018

1 to 5 years

year

<0.001

<0.001

Lead

95

2017

2018

1 to 17 years

age

0.003

<0.001

Lead

95

2015

2018

1 to 5 years

income

0.285

<0.001

Lead

95

1988

2018

1 to 5 years

year

<0.001

<0.001

Lead

95

1999

2018

1 to 5 years

year

<0.001

<0.001

* For Against = "age" the comparison is between the age groups 1, 2, 3-5, 6-10, 11-15 and 16-17 years, and the p-values are adjusted for sex, race/ethnicity, and income.

For Against = "income," the comparison is between those below the poverty level and those at or above the poverty level, and the p-values are adjusted for age, sex, and race/ethnicity.

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For Against = "year" the comparison is the trend over different years, and the p-values are adjusted for age, sex, race/ethnicity, and income.

Table 4. Statistical significance tests comparing the percentiles of blood lead levels in children ages 1 to 5 years between 1991-1994
and 2015-2018.



P-VALUES

Variable

Percentile

From

To

Against

Subset*

Unadjusted

Adjusted**

Lead

50

1991-1994

2015-2018

year



<0.001

<0.001

Lead

50

1991-1994

2015-2018

year

White non-Hispanic

<0.001

<0.001

Lead

50

1991-1994

2015-2018

year

Black non-Hispanic

<0.001

<0.001

Lead

50

1991-1994

2015-2018

year

Mexican American

<0.001

<0.001

Lead

50

1991-1994

2015-2018

year

Other

<0.001

<0.001

Lead

95

1991-1994

2015-2018

year



<0.001

<0.001

Lead

95

1991-1994

2015-2018

year

White non-Hispanic

<0.001

<0.001

Lead

95

1991-1994

2015-2018

year

Black non-Hispanic

<0.001

<0.001

Lead

95

1991-1994

2015-2018

year

Mexican American

<0.001

<0.001

Lead

95

1991-1994

2015-2018

year

Other

<0.001

<0.001

* "Other" represents the "All Other Races/Ethnicities" category, which includes all other races and ethnicities not specified, together with those individuals who report more than one race.

** For Against = "year" where Subset is missing, the comparison is between the different periods, and the p-values are adjusted for age, sex, race/ethnicity, and income.

For Against = "year" where Subset is not missing, the comparison is between different periods for each race/ethnicity group , and the p-values are adjusted for age, sex, and income.

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Data Files

The following files are needed to calculate these indicators. All these files together with the
survey documentation and SAS® programs for reading in the data are available at the NHANES
website: www.cdc.gov/nchs/nhanes.htm. As an exception, the blood lead data for children ages 1
to 5 years for 2017-2018 is not available publicly; access may be obtained through the NCHS
Research Data Center.

•	NHANES II: Hematology and Biochemistry file DU5411 .txt. This text file contains the
measured blood lead (N2LB0409), age in years at screening (N2LB0047), sex
(N2LB0055), race/ethnicity (N2LB0060), poverty income ratio (N2LB0210), the lead
final examined weight (N2LB0300), the pseudo-stratum (N2LB0324), and the pseudo-
PSU (N2LB0326).

•	NHANES III: Laboratory file LAB.DAT. This text file contains the measured blood lead
(PBP), age in years at screening (HSAGEIR), sex (HSSEX), race/ethnicity
(DMARETHN), poverty income ratio (DMPPIR), NHANES III Phase (SDPPHASE), the
Final Examination (Mobile Examination Center (MEC) and Home) weights (WTPFHX1
for Phase I, WTPFHX2 for Phase 2, WTFHX6 for Total sample), the pseudo-stratum
codes (SDPSTRA1 for Phase 1, SDPSTRA2 for Phase 2, and SDPSTRA6 for Total
sample), and the pseudo-PSU codes (SDPPSU1 for Phase 1, SDPPSU2 for Phase 2, and
SDPPSU6 for Total sample).

•	NHANES 1999-2000: Demographic file demo.xpt. Laboratory file lab06.xpt. The
demographic file demo.xpt is a SAS® transport file that contains the subject identifier
(SEQN), age (RIDAGEYR), sex (RIAGENDR), race/ethnicity (RIDRETH1), poverty
income ratio (INDFMPIR), pseudo-stratum (SDMVSTRA), pseudo-PSU (SDMVPSU),
and the two-year MEC weight (WTMEC2YR). The laboratory file lab06.xpt contains
SEQN and the blood lead (LBXBPB). The two files are merged using the common
variable SEQN.

•	NHANES 2001-2002: Demographic file demo_b.xpt. Laboratory file 106_b.xpt. The
demographic file demo b.xpt is a SAS® transport file that contains the subject identifier
(SEQN), age (RIDAGEYR), sex (RIAGENDR), race/ethnicity (RIDRETH1), poverty
income ratio (INDFMPIR), pseudo-stratum (SDMVSTRA), pseudo-PSU (SDMVPSU),
and the two-year MEC weight (WTMEC2YR). The laboratory file 106_b.xpt contains
SEQN and the blood lead (LBXBPB). The two files are merged using the common
variable SEQN.

•	NHANES 2003-2004: Demographic file demo_c.xpt. Laboratory file 106bmt_c.xpt. The
demographic file demo c.xpt is a SAS® transport file that contains the subject identifier
(SEQN), age (RIDAGEYR), sex (RIAGENDR), race/ethnicity (RIDRETH1), poverty
income ratio (INDFMPIR), pseudo-stratum (SDMVSTRA), pseudo-PSU (SDMVPSU),
and the two-year MEC weight (WTMEC2YR). The laboratory file 106bmt_c.xpt contains

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SEQN and the blood lead (LBXBPB). The two files are merged using the common
variable SEQN.

•	NHANES 2005-2006: Demographic file demod.xpt. Laboratory file pbcdd.xpt. The
demographic file demo d.xpt is a SAS® transport file that contains the subject identifier
(SEQN), age (RIDAGEYR), sex (RIAGENDR), race/ethnicity (RIDRETH1), poverty
income ratio (INDFMPIR), pseudo-stratum (SDMVSTRA), pseudo-PSU (SDMVPSU),
and the two-year MEC weight (WTMEC2YR). The laboratory file pbcd d.xpt contains
SEQN and the blood lead (LBXBPB). The two files are merged using the common
variable SEQN.

•	NHANES 2007-2008: Demographic file demo_e.xpt. Laboratory file pbcd_e.xpt. The
demographic file demo e.xpt is a SAS® transport file that contains the subject identifier
(SEQN), age (RIDAGEYR), sex (RIAGENDR), race/ethnicity (RIDRETH1), poverty
income ratio (INDFMPIR), pseudo-stratum (SDMVSTRA), pseudo-PSU (SDMVPSU),
and the two-year MEC weight (WTMEC2YR). The laboratory file pbcd e.xpt contains
SEQN and the blood lead (LBXBPB). The two files are merged using the common
variable SEQN.

•	NHANES 2009-2010: Demographic file demo f.xpt. Laboratory file pbcd f.xpt. The
demographic file demof.xpt is a SAS® transport file that contains the subject identifier
(SEQN), age (RIDAGEYR), sex (RIAGENDR), race/ethnicity (RIDRETH1), poverty
income ratio (INDFMPIR), pseudo-stratum (SDMVSTRA), pseudo-PSU (SDMVPSU),
and the two-year MEC weight (WTMEC2YR). The laboratory file pbcdf.xpt contains
SEQN and the blood lead (LBXBPB). The two files are merged using the common
variable SEQN.

•	NHANES 2011-2012: Demographic file demo_g.xpt. Laboratory file pbcd_g.xpt. The
demographic file demo g.xpt is a SAS® transport file that contains the subject identifier
(SEQN), age (RIDAGEYR), sex (RIAGENDR), race/ethnicity (RIDRETH1), poverty
income ratio (INDFMPIR), pseudo-stratum (SDMVSTRA), pseudo-PSU (SDMVPSU),
and the two-year MEC weight (WTMEC2YR). The laboratory file pbcd g.xpt contains
SEQN and the blood lead (LBXBPB). The two files are merged using the common
variable SEQN.

•	NHANES 2013-2014: Demographic file demoh.xpt. Laboratory file pbcdh.xpt. The
demographic file demo h.xpt is a SAS® transport file that contains the subject identifier
(SEQN), age (RIDAGEYR), sex (RIAGENDR), race/ethnicity (RIDRETH1), poverty
income ratio (INDFMPIR), pseudo-stratum (SDMVSTRA), pseudo-PSU (SDMVPSU),
and the two-year MEC weight (WTMEC2YR). The laboratory file pbcd h.xpt contains
SEQN, blood lead (LBXBPB), and the blood metal weight (WTSH2YR). The two files
are merged using the common variable SEQN. The blood metal weight is used to adjust
the survey to the national populations, accounting for the fact that the data were a full
sample of participants ages 1 to 5 but a one-half subsample of participants ages 12 years
and over.

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•	NHANES 2015-2016: Demographic file demoi.xpt. Laboratory file pbcdi.xpt. The
demographic file demo i.xpt is a SAS® transport file that contains the subject identifier
(SEQN), age (RIDAGEYR), sex (RIAGENDR), race/ethnicity (RIDRETH1), poverty
income ratio (INDFMPIR), pseudo-stratum (SDMVSTRA), pseudo-PSU (SDMVPSU),
and the two-year MEC weight (WTMEC2YR). The laboratory file pbcd i.xpt contains
SEQN, blood lead (LBXBPB), and the blood metal weight (WTSH2YR). The two files
are merged using the common variable SEQN. The blood metal weight is used to adjust
the survey to the national populations, accounting for the fact that the data were a full
sample of participants ages 1 to 5 but a one-half subsample of participants ages 12 years
and over.

•	NHANES 2017-2018: Demographic file demoj.xpt. Laboratory file pbcdj.xpt. Blood
Lead data for youth aged 1-5 years are included in file "Lead - Blood - Youth
(PBYJR)", and available through the NCHS Research Data Center. The demographic
file demoj.xpt is a SAS® transport file that contains the subject identifier (SEQN), age
(RIDAGEYR), sex (RIAGENDR), race/ethnicity (RIDRETH1), poverty income ratio
(INDFMPIR), pseudo-stratum (SDMVSTRA), pseudo-PSU (SDMVPSU), and the two-
year MEC weight (WTMEC2YR). The laboratory file pbcd j .xpt contains SEQN and the
blood lead (LBXBPB). The files PBY J R and pbcdj.xpt are combined into one file.
The combined lead data file and demographic file are merged using the common variable
SEQN.

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